I need to travel from Lhasa (拉萨) to Xi'an (西安) in early April 2018, and I'm thinking of travelling by train as the Qinghai-Tibet Railway is said to be spectacular. I particularly want to see the area around the Tanggula Pass, which at 5072m is the world's highest section of railway and is supposed to be particularly impressive, so I'd like to be sure I don't choose a train which passes through this area at night. The problem is, I can't work out when the trains pass Tanggula. There is a station at Tanggula, but either the trains don't stop there or it's too minor to appear in the timetables I've found. This timetable tells me that Nagqu / Nakchu (那曲) is 3½–4h out of Lhasa, and Golmud / Ge'ermu (格尔木) is 14–15h out. According to the list of stations on Wikipedia, Tanggula is about a third of the way from Nagqu to Golmud which would suggest it's reached about 7–8h after leaving Lhasa. If that's right then any morning departure should be okay as sunset will be about 20:10. But with so much elevation to gain, the climb up to Tanggula may well take disproportionately long. Is anyone able to help find a more detailed timetable that might address my concern?
I think you should be fine. The nearest manned station to Tanggula is Anduo (Amdo, 安多), and that's only a 5 hour train ride. It's at most another hour from Amdo to Tanggula, so I'd say 6 hours is reasonable.
Also, I want to just say the scenery of that part of the train ride is actually fairly unremarkable. The train doesn't stop there, and the land's pretty barren, so don't set your expectations too high! The train ride itself actually is really interesting if you haven't been on a developing-country sleeper, though, so that'll be quite the highlight.